This invention relates to fuel injection pumps for diesel engines, and in particular to improvements in a Bosch fuel injection pump.
In a diesel fuel injection system, as is well known, the fuel is injected under pressure into the combustion chamber toward the end of the compression stroke. The heat produced by compressing the air ignites the injected fuel, eliminating the need for spark plugs or a separate ignition system. The Bosch fuel injection pump has found extensive use in such injection systems for metering the fuel and delivering it under pressure to the nozzle or nozzles at the diesel engine cylinder or cylinders. It includes a cam driven plunger reciprocably mounted within a barrel to define a pumping chamber. A fuel inlet port or ports are open to this pumping chamber in such a position that, on its power stroke, the plunger travels some distance before it covers the fuel inlet port or ports, and then starts pressurizing the fuel that has been confined in the pumping chamber. This distance between the extreme plunger position at the start of the power stroke and the intermediate position where the plunger covers the fuel inlet port or ports is referred to as the prestroke of the plunger. The prestroke has so far been fixed.
This type of fuel injection pump has had some weaknesses. At low engine speed the plunger is driven at correspondingly low speed, so that the fuel has been delivered to the combustion chamber under low pressure. The fuel pressure has also been low when the engine is under light load, demanding a small amount of oil, because then, such being the contour of the drive cam, the plunger is traveling at low speed when it completes the fuel injection. Combustion efficiency has thus been relatively low under these conditions.
The prior art construction of the fuel injection pump has required a sepatate injection timing mechanism of very complex design. The complex timing mechanism has added substantially to the manufacturing cost of the diesel fuel injection system.